Delegates And Events – Part 2 of 3

19. November 2020 12:42 by Parakh in Visual C#  //  Tags: ,   //   Comments
This post explains the concept of delegates in a lucid manner, giving simple to understand code examples.

In my last post, I explained the concept of events, delegates, and event handlers at a high level. In this post, let’s take a look at delegates. Why delegates and not events, because among them, they are the tricky ones and can be understood in relative isolation.

Delegates form the glue connecting the events and event handlers and are instrumental in conveying to all the subscribed event handlers about the firing of an event. What I will try to show in this post are the following:
1. High-level conceptual coverage of delegates,
2. Technical overview of delegates,
3. How a delegate can execute a connected method, aka an event handler

High-Level Overview:

Consider the following scenario where you have a pipeline going out from a pumping station and connected to that pipeline are houses and industries. Houses will use the supplied water for cleaning, cooking, etc. while the industries will use the same water for chemical and other manufacturing purposes. Water Pumping station supplying water to homes and industries


 

The pumping station here is synonymous with an event firing when an action of interest takes place. The water is the information conveyed by the event, and the houses and industries are the event handlers which process the supplied water, aka the information in the desired form. Delegates form the plumbing connecting the events and event handlers.

Before we move any further, we must understand what is a callback mechanism.

Callback Mechanism

Essentially, a callback mechanism that lets us chain pieces of code together in such a way that the first piece calls the second. The call can be made in two ways:

1) Synchronous callbacks, in which the first code (calling code) waits for the second code (called code) to finish the work and deliver the result, and, only after that resumes execution.
E.g. You can have a windows form application which has a button which upon clicking starts execution of a long running process. While the process is running the windows form will remain unresponsive.

2) Asynchronous callbacks, where the first code calls the second code but does not wait for it to finish the work. The work by the second piece of code is carried out on a different thread.

Callback mechanism is known by different names in different programming languages. In C, it is known as function pointers, and in C# it is called “delegates”.

In C#, delegates form the plumbing that allows one code to call another code synchronously or asynchronously. Adding to that is the fact that they are type-safe, i.e. you can only call methods that comply with the delegate’s signature and return type. In this post we will focus only on the synchronous calling via delegates

Use cases for delegates

Delegates can be used in two scenarios:

1. To broadcast changes in an object’s state
2. To broadcast fulfillment of a condition

Delegates behind the scenes are a special type of class, which means that we need to create an object of delegate type to use it for callback purposes.

Let’s dive into practical examples that show the use cases in action.

Broadcast changes in an object’s state


Consider human beings and their sensitiveness to ambient temperature. We can create a class “Human” and broadcast the response of its object to different temperatures.

public class Human
    {
        // Basic properties
        public int Id { get; set; }
        public string Name { get; set; }
        public float MinTemp { get; set; }
        public float MaxTemp { get; set; }
 
        //Delegate declaration
        public delegate void TemperatureSensationHandler(string message);
 
        // Member variable of the declared delegate
        public TemperatureSensationHandler temperatureSensationMembers;
 
        public Human()
        {
        }
 
        // User defined constructor
        public Human(int id, string name, float minTemp, float maxTemp)
        {
            Id = id;
            Name = name;
            minTemp = MinTemp;
            maxTemp = MaxTemp;
        }
 
        public void TemperatureSensation(float temperature)
        {
            if (temperatureSensationMembers!= null)
            {
                if (temperature > MaxTemp)
                {
                    temperatureSensationMembers("I am feeling hot");
                }
                else if (temperature < MinTemp)
                {
                    temperatureSensationMembers("I am feeling cold");
                }
                else
                {
                    temperatureSensationMembers("I am feeling normal");
                }
            }         
        }
    }

 

In the code above, we have a class "Human" with four properties, of which two are of importance - MinTemp and MaxTemp. The method TemperatureSensation accepts a float type parameter and depending upon where the value stands in the spectrum between MinTemp and MaxTemp, broadcasts an appropriate message. In the following code, we create an object of the Human class and test the functionality of delegate declared in the Human class.

class Program
    {
        static void Main(string[] args)
        {
            Human human = new Human() 
{ Id = 1, Name = "Parakh", MinTemp = 25.5f, MaxTemp = 45.0f };
 
            human.temperatureSensationMembers += OnTemperatureSensationEvent;
            human.TemperatureSensation(100);
            human.TemperatureSensation(30);
            human.TemperatureSensation(10);
            Console.ReadLine();
        }
 
        public static void OnTemperatureSensationEvent(string message)
        {
            Console.WriteLine(message);
        }
    }

 

In the code above we created an object of the Human class and instantiated it with some data. Then, we enrolled in the delegate object a method that matched the delegate’s signature and return type. Then we invoked the TemperatureSensation method on the human object which then broadcasted the messages to the delegate’s subscribed members.

The above code when executed yields the following outputBoradcast changes in an object's state

Now there’s one flaw in the code above. Delegate class at the .Net framework level overrides the “+” and “-“ operators, allowing the subscription and unsubscription of complying methods. It means that having direct access to the delegate object is sensitive and a developer should not be able to empty a delegate object’s invocation list by using the “-“ operator repeatedly.

We thus encapsulate the subscription and unsubscription of the methods to the delegate object via methods.

public class Human
    {
        // Basic properties
        public int Id { get; set; }
        public string Name { get; set; }
        public float MinTemp { get; set; }
        public float MaxTemp { get; set; }
 
        //Delegate declaration
        public delegate void TemperatureSensationHandler(string message);
 
        // Member variable of the declared delegate
        private TemperatureSensationHandler temperatureSensationMembers;
 
        //Registration and un-registration methods
        public void RegisterTempSensationHandlerMethods(TemperatureSensationHandler methodToRegister)
        {
            temperatureSensationMembers += methodToRegister;
        }
 
        public void UnRegisterTempSensationMethods(TemperatureSensationHandler methodToUnregister)
        {
            if (temperatureSensationMembers != null)
            {
                temperatureSensationMembers -= methodToUnregister;
            }
        }
 
        public Human()
        {
 
        }
 
        // User defined constructor
        public Human(int id, string name, float minTemp, float maxTemp)
        {
            Id = id;
            Name = name;
            minTemp = MinTemp;
            maxTemp = MaxTemp;
        }
 
        public void TemperatureSensation(float temperature)
        {
            if (temperatureSensationMembers!= null)
            {
                if (temperature > MaxTemp)
                {
                    temperatureSensationMembers("I am feeling hot");
                }
                else if (temperature < MinTemp)
                {
                    temperatureSensationMembers("I am feeling cold");
                }
                else
                {
                    temperatureSensationMembers("I am feeling normal");
                }
            }         
        }
    }

With the modified code, we now can register and un-register the methods without exposing the delegate object to a developer.

The code above yields the same output.

Boradcast changes in an object's state

Broadcast of fulfilment of a condition


Some conditions warrant broadcast of information when they materialize. Examples would include intimation upon sending or receiving a message, dispatch, and delivery of a physical item in case of e-commerce applications, etc.

Here we will take an example of checking out a book from a library.

We have four classes - Book, Member, Author, and Library. The idea in this example is to broadcast a message upon execution of certain activities like searching for a book, searching for member records, and successful or unsuccessful checking out of a book.

public class Author
   {
        public int AuthorId { get; set; }
        public string FirstName { get; set; }
        public string LastName { get; set; }
    }
 
public class Book
    {
        public int BookId { get; set; }
        public string ISBN { get; set; }
        public string Title { get; set; }
        public Author Author { get; set; }
        public int PageCount { get; set; }
        public bool IsCheckedOut { get; set; }
    }
 
public class Member
    {
        public int MemberId { get; set; }
        public string FirstName { get; set; }
        public string LastName { get; set; }
        public int NumberOfBooksCheckedOut { get; set; }
        public List<Book> BooksCheckedOut { get; set; }
    }
 
    public class Library
    {
        public int LibraryId { get; set; }
        public List<Book> Books { get; set; }
        public List<Member> Members { get; set; }
        public Library()
        {
            LibraryId = 1;
            Books = new List<Book>()
                        {
                            new Book()
                            {
                                BookId = 1, 
                                Title = "Alice in Wonderland", 
                                IsCheckedOut = false, 
                                PageCount = 200,
                                Author = new Author() 
                                { 
                                    AuthorId = 1, 
                                    FirstName = "Lewis", 
                                    LastName = "Carroll"
                                }
                            },
                            new Book()
                            { 
                                BookId = 2, 
                                Title = "Bad Blood", 
                                IsCheckedOut = false, 
                                PageCount = 350,
                                Author = new Author()
                                { 
                                    AuthorId = 2, 
                                    FirstName = "John", 
                                    LastName = "Carreyrou"
                                }
                            },
                            new Book()
                            { 
                                BookId =  3, 
                                Title = "The Dream Machine", 
                                IsCheckedOut = false, 
                                PageCount = 250,
                                Author = new Author()
                                { 
                                    AuthorId = 3, 
                                    FirstName="Mitchell", 
                                    LastName = "Waldrop"
                                }
                            },
                            new Book()
                            { 
                                BookId = 4, 
                                Title = "The Structure of Scientific Revolution", 
                                IsCheckedOut = false,
                                PageCount = 500, 
                                Author = new Author()
                                { 
                                    AuthorId = 4, 
                                    FirstName = "Thomas", 
                                    LastName=  "Kuhn"
                                }
                            },
                            new Book()
                            { 
                                BookId =5,
                                Title = "Sapiens: A Brief History of Humankind", 
                                IsCheckedOut = false,
                                PageCount = 450, 
                                Author =new Author() 
                                { 
                                    AuthorId = 5, 
                                    FirstName = "Yuval", 
                                    LastName = "Hariri"
                                }
                            }
                        };
            Members = new List<Member>()
                        {
                            new Member()
                            { 
                                MemberId = 1, 
                                FirstName = "Parakh", 
                                LastName = "Singhal", 
                                NumberOfBooksCheckedOut = 0
                            },
                            new Member()
                            { 
                                MemberId = 2, 
                                FirstName = "Prateek", 
                                LastName = "Mathur", 
                                NumberOfBooksCheckedOut = 0
                            },
                            new Member()
                            { 
                                MemberId =3, 
                                FirstName = "Sumant", 
                                LastName = "Sharma", 
                                NumberOfBooksCheckedOut = 0
                            }
                        };
 
        }
 
        //Delegate which can accept methods having a string parameter
        public delegate void CheckoutHandler(string message);
 
        private CheckoutHandler checkOutHandlerMembers;
 
        public void RegisterCheckOutHandlerMembers(CheckoutHandler methodToRegister)
        {
            checkOutHandlerMembers += methodToRegister;
        }
 
        public void UnRegisterCheckOutHandlerMembers(CheckoutHandler methodToUnregister)
        {
            if (checkOutHandlerMembers != null)
            {
                checkOutHandlerMembers -= methodToUnregister;
            }
        }
 
        public void CheckOutBook(int MemberId, int bookId)
        {
            //The operation of search will be printed out by the program only 
            //if there are members to the delegate type
            if (checkOutHandlerMembers != null)
            {
                checkOutHandlerMembers("Searching for book and Member records");
            }
 
            Book bookToBeCheckedOut = Books.Find(book => book.BookId == bookId);
            Member MemberCheckingOut = Members.Find(Member => Member.MemberId == MemberId);
 
            //The operation of success in finding the book and Member will
            //only be printed if there are members to the delegate type
            if (bookToBeCheckedOut != null &&
                MemberCheckingOut != null && 
                checkOutHandlerMembers != null)
            {
                checkOutHandlerMembers("Book and Member records found.");
            }
 
            if (bookToBeCheckedOut.IsCheckedOut == false)
            {

//The final result, whether the book can be checked

//out needs to be printed whether

                //there are any members to the delegate type. 
                //That's why if-else statement with duplicate
                //console writes
                if (checkOutHandlerMembers != null)
                {
                    checkOutHandlerMembers("Congratulations, the book is available");
                }
                else
                {
                    Console.WriteLine("Congratulations, the book is available");
                }
            }
            else
            {
                if (checkOutHandlerMembers != null)
                {

checkOutHandlerMembers("Apologies, but the book is already

checked out to an existing Member.");

                }
                else
                {

Console.WriteLine("Apologies, but the book is

already checked out to an existing Member.");

                }
            }
        }
    }
}

 

The model created above is consumed in the console application.

class Program
    {
        static void Main(string[] args)
        {
            Library library = new Library();
            library.RegisterCheckOutHandlerMembers(OnCheckingOutEvent);            
            library.CheckOutBook(1, 1);
            
            Console.ReadLine();
        }
 
        public static void OnCheckingOutEvent(string message)
        {
            Console.WriteLine(message);
        }
    }

 

When executed the following output appears:

Broadcast of fulfillment of a conditionIn the output shown above, the program emitted certain messages related to the searching of the member and book details and successful checkout of the book. Now, remember, the domain is responsible for emitting the information, but what we do with the information is entirely in the hands of the developer. We can compose an email or an SMS and embed the emitted message or have it logged in a log file or a database.

Summary

Delegates help in separating the act of broadcasting information and how that information is consumed.
I hope this post helped in laying a strong foundation by presenting the concept of delegates in an easy to understand manner.

References

  1. Pro C# 8 with .NET Core 3 by Andrew Troelsen and Phil Japikse
  2. CLR via C# by Jeffrey Richter

Delegates And Events–Part 1 of 3

19. November 2020 09:45 by Parakh in Visual C#  //  Tags: , ,   //   Comments
This post attempts at explaning the concept of delegates and events in C# as it relates to real life, in as simple a manner as possible.

In my experience, if there’s one topic in C# that baffles the programmers the most, it is the topic of events and delegates. And the root of the state of confusion is not the lack of attempt on the programmer’s part, but the way the books on programming languages explain the concept – in a dry academic sense, devoid of any relatability to real life.

This blog post is my attempt to explain the concept of events and delegates in an easy way.

Events in real life are occurrences where something of importance happened to an entity. Let me elaborate on that. Consider a normal human being. Events in the life of a normal human being would be:
1. Birth
2. First day of school
3. Graduation from school
4. First day of college
5. Graduation from college
6. First pay-check
7. Getting married
8. First child
…and so on. The aforementioned occurrences would normally be considered important, hence endowed with a special term, events.

Whenever an event occurs, there’s something done. Birth of a child is celebrated, graduation from school is celebrated with a farewell party, marriages are celebrated with near and dear ones, etc.

This is what is the inspiration for the abstraction, aptly called events in modern object-oriented programming languages. Events are important occurrences in the life-span of an entity. An entity can be a button in a Windows Form, a dropdown on a web page, or some kind of a custom activity in a user-defined object like a domain class. Correspondingly, the event would be pressing a button, changing the selected element in a dropdown or value of a property in a domain class.

Whenever some event occurs or gets fired, something is done. That something is done in some defined code, called the event handler. An event handler is essentially a method, with details of what needs to be done when the event is fired.

Now that we have understood the concept of events and event handlers, let’s understand the concept of delegates. Delegates are like the consciousness that pushes us to do something when an event occurs. Essentially, a delegate is the glue that ties an event and an event handler together.

To summarise:
When something of importance happens -> Event
What we do in case of an event -> Event Handler
The glue connecting the event and event handler –> Delegate

I hope that this post was able to sow the idea of events and delegates your mind in a non-technical and relatable way. In future posts, I will shed more light on the topic.

How to Create and Apply Git Patch Files

2. November 2019 09:20 by Parakh in Git  //  Tags: , , ,   //   Comments
This post explains how to create patch files using the git format-patch command and then apply them using the apply and am commands.

Introduction

In the years during which Git evolved, there was no provision of using a central remote repository which different contributors could use to merge their changes to a feature branch or request pull requests. Sending patch file(s) via email was the name of the game. A patch file is a handy way of encapsulating the changes introduced into the repository as part of a commit, into a single file. Patches can be created on a per commit basis or a bunch of commits can be squashed into a single patch file.

Git Format-Patch

The command that is used to create a patch file is format-patch. Various options are available that can be used in conjunction with the command to modify the output as desired. Let’s take a look at the most common operations that use format-patch command.

To learn about the command let’s create a repository and track a text file with some text entries.

$ mkdir PatchFiles
$ cd PatchFiles
$ git init
$ touch file1.txt
$ printf "first line" > file1.txt
$ git add .
$ git commit -m "First commit"
$ printf "\nsecond line" >> file1.txt
$ git commit -am "Second commit"
$ git log --online master

Creation of master branch

Next let’s create a branch named develop off of the master branch and track a text file with some text entries.

$ git branch develop
$ git checkout develop
$ touch file2.txt
$ printf "first line" > file2.txt
$ git add .
$ git commit -m "First commit in develop branch"
$ printf "\nsecond line" >> file2.txt
$ git commit -am "Second commit in develop branch"
$ git log --oneline

Creation of develop branch

Now that we have laid the groundwork, let’s understand the format-patch command. The format-patch command takes in the name of the branch against which you want to compare the commits of the current branch pointed to by the INDEX pointer. By default, the format-patch creates a patch file for every commit not available in the target branch.

$ git format-patch master
$ dir

Format Patch Multiple files

In earlier times, when there was no remote repository, such patch files used to be mailed by project contributors to the project maintainer.
Next, we will discuss the two commands that can be used to apply the patch files, i.e., apply and am.

Git Apply

There could be situations that demand the application of changes in a patch to the target branch, but, not include the corresponding commit message into the commit history of the target branch. Git’s apply command is used in such situations.

Apply command helps you to test out the changes introduced by the patch locally before you formally commit them into the repository. Since the changes are applied to the working directory, you can use git diff to view the changes applied. Also, the commit message can be a bit more descriptive signifying the nature of the changes applied.

Let’s see the command in action in code.

$ git checkout master
$ git apply 0001-First-commit-in-develop-branch.patch
$ git status
$ git log --oneline
$ dir

 05 Apply command - cropped

Git AM

Git’s am (Apply Mailbox) command is used in a situation when you are assured of the commits in a patch and want them to be applied verbatim, along with their corresponding metadata consisting of author’s information and commit message.

$ git checkout master
$ git am 0001-First-commit-in-develop-branch.patch
$ git status
$ git log --oneline
$ dir

06 AM command - cropped

Word of Caution

If you are selectively applying patches, and if a patch contains an artifact that comes to life in a prior patch, then the application of such a patch will fail. This is because patch files are commit-specific and carry changes specific to the commit.
In such cases, you will be required to sequentially apply patches in chronological order, first bringing the artifact into existence, followed by any changes in the artifact.

$ git checkout master
$ git am 0002-Second-commit-in-develop-branch.patch

07 Problem with patch application - cropped

 

We were not able to successfully apply the second patch as the file2.txt came into existence in the first commit, hence first patch. It would be imperative here, to first apply the first patch and then the second patch.

Conclusion

Git provides the ability to create patches as discreet units signifying the evolution of a project and enables the sharing of this evolution in the absence of a central remote repository.

Git offers two ways to apply patches as per requirements.

Git’s apply command applies the changes locally to the working directory and gives you the freedom to further introduce changes in the artifacts before formally committing into the repository.

Git’s am command applies the patches in the repository and includes the commit messages contained in the patches. It retains vital metadata like author information, email, etc..

A short note on Git Cherry-Pick

16. October 2019 13:27 by Parakh in Git, GitHub  //  Tags: ,   //   Comments
A short note demonstrating git&#39;s cherry-pick command.

Key Takeaway

In the real world, situations arise where some urgent work is done on a codebase that is required to be released to mainstream users. This work then needs to be bought down into the feature branches to maintain parity with the master branch.

In Git, we have the command of Rebase to handle such a situation, but in some cases, a more surgical approach is required, whereby, only the commit having the changes is required to be applied to the feature branch and exclude anything else which might otherwise pollute the feature branch. This is what Git’s cherry-pick command achieves.

Read On

One of the founding premises of Git is to commit often and commit early. This aspect of the foundation enables you, the user, to see the evolution of your work over time and enables Git to take the responsibility to manage the evolution in a discreet order via commits. By having small commits, you enable Git to recover your work in a granular manner, should the situation come to be, as desired. And dovetailing this practice of small and frequent discreet commits, revolves the power of the command of cherry-pick.

Git’s cherry-pick command helps you apply a commit in a surgical manner such that it only brings in the changes corresponding to a commit to the branch under concern. This command can be thought to be useful in scenarios whereby you have to bring in a limited number of changes that are pertinent to your work. Examples:
1. There’s a hotfix branch created from the master branch and you need to include the hotfix in your feature branch.
2. There are a couple of commits that you need to take from some other feature branch and incorporate in your branch

Let’s take a look at an example to understand the cherry-pick command.

$ mkdir cherrypick
$ cd cherrypick
$ git init
$ touch file1.txt
$ printf "first line" > file1.txt
$ git add .
$ git commit -m "First commit in master branch"
$ printf "\nsecond line" >> file1.txt
$ git commit -am "Second commit in master branch"
$ printf "\nthird line" >> file1.txt
$ git commit -am "Third commit in master branch"
$ git branch develop
$ git checkout develop
$ touch file2.txt
$ printf “first line” >> file2.txt
$ git add .
$ git commit -m “First commit in develop branch”
$ printf “\nsecond line” >> file2.txt
$ git commit -am “Second commit in develop branch”
$ printf “\nthird line” >> file2.txt
$ git commit -am “Third commit in develop branch”
$ git checkout master
$ git log --oneline develop
$ git cherry-pick SHA1 hash of the second commit from the develop branch

 

Resolve any conflicts that arise from the cherry pick here and then run a git log

$ git log --oneline master

 

In this example, we created a directory aptly named cherrypick and then inserted some text entries into a text file named file1.txt. We then created a branch called develop from the master branch. In the develop branch we created some entries in a text file named file2.txt and then moved back to the master branch. Now listed the log entries from the develop feature branch and noted down the SHA1 hash of the commit corresponding to which we want to pick the changes from the develop branch. In this case, we picked the second commit from the develop branch.

01 Creation of repoFigure 1 Creating the repository and some initial commits

02 In Develop BranchFigure 2 Creation of a branch and some initial commits

03 In Develop BranchFigure 3 Going back to the branch where a cherry-picked commit needs to be merged

Finally, we used Git’s cherry-pick command in the master branch in conjunction with the SHA1 hash noted before. This will bring in the changes from the develop branch into the master branch that were made corresponding to the commit hash. In doing so, you may be required to resolve any conflict that may arise.

04 In Master BranchFigure 4 Process of cherry picking may throw up some conflicts which would require intervention

05 In Master BranchFigure 5 Once a commit is cherry picked, its content will then become available in the target branch

This way we can literally cherry-pick the changes corresponding to specific commit(s) in a branch and bring them to the branch of our choice.

Hope this was helpful.

Git Revert vs. Reset

5. October 2019 13:20 by Parakh in Git  //  Tags: , ,   //   Comments
This post explains the differences between the git commands of revert and reset and where to use them. The post also shows the three different types of reset types.

Introduction

We all make mistakes, and, it’s my belief that we all make an honest attempt to correct that mistake. If only, life was running on Git. Git offers two ways to rollback a change made to a codebase – revert and rese. In this article we will discuss the two commands and the scenarios in which they are apt to be used.

Basic Terminology

Before we embark on discussing the differences between the commands, it is prudent to discuss some basic terminology that will be instrumental in understanding the nuances of operations offered by these commands.
HEAD: Represents the last commit and is a pointer to the current branch selected in git.
INDEX: Represents the staging area aka the contents of the proposed next commit.
Working Directory: Contains all the files available in HEAD and INDEX and the artifacts that may never go in any commit. E.g. Assembly files (.dll, .exe) which do not get committed due to .gitignore settings.

Revert command

Sometimes it happens that wrong work gets committed and is pushed into the remote repository, making it available to the rest of the team, or project contributors, as the case may be. In such circumstances, it becomes imperative, that we introduce a context under which a rollback of the unintentional changes take place.

Revert offers just such functionality. It creates an opposite commit of the commit that you want to roll back. Such a rollback commit retains a detailed history of all the commits that came before it, thus avoiding all the confusion.

Run the following commands in bash shell to simulate a revert:

$ mkdir Revert
$ cd Revert
$ git init
$ touch file.txt
$ printf “First commit”>file.txt
$ git add file.txt
$ git commit -m “First commit”
$ touch file2.txt
$ printf “Second commit”>file2.txt
$ git add file2.txt
$ git commit -m “Second commit”
$ printf “\n\nThird commit”>>file.txt
$ git add file.txt
$ git commit -m “Third commit”
$ dir

Note that there are two files file.txt and file2.txt in the INDEX and working directory.

$ git status
$ git log –oneline

Note there are three commits. Now let’s revert the second commit. Since the file2.txt was created as part of the second commit, it should get deleted from the working directory and INDEX

$ git revert SHA1 hash corresponding to the second commit.
$ dir
$ git log –oneline
$ git status

The result of following the aforementioned commands should look something like:

01 Revert Commit Log Cropped

As you see, now we have a total of four commits in the history, with the fourth one clearly jotted down as a revert commit, erasing the work done as part of the second commit.

Reset command

Reset is used when you have the fault available locally on your system and has not been pushed out to the remote repository. Now this command offers a further three flavors and it will be here that we will be chiefly leveraging the terminology covered earlier.
Soft Reset: This option moves the branch (that HEAD points to) to the commit mentioned, reinstating the conditions as they were when the commit was made. No changes are introduced in the INDEX (staging area) and working directory. This essentially does the same this as “git commit –amend” and offers us a chance to change what we need to.
This means that you will get a chance to amend the commit message. Note that no changes are introduced into the file(s) made prior to the rolled back commit in soft mode.
Mixed Reset: This option moves the branch (that HEAD points to) and INDEX (staging area) to the commit mentioned, reinstating the conditions as they were when the commit was made. No changes are introduced in the working directory.
Hard Reset: This option moves the branch (that HEAD points to), INDEX (staging area) and the working area to the commit mentioned, reinstating the conditions as they were when the commit was made. This is destructive in nature and should be used with caution as work is not recoverable after the issuance of this command.

NOTE: In none of the cases HEAD is moved to the mentioned commit. HEAD always point to the branch. It is the branch that is made to point to the pointer corresponding to the commit being reset.

Run the following commands in bash shell to simulate all three flavors of reset:

$ mkdir Reset
$ cd Reset
$ git init
$ touch file.txt
$ printf “First commit” > file.txt
$ git add file.txt
$ git commit -m “First commit”
$ printf “\n\nSecond commit” >> file.txt
$ git add file.txt
$ git commit -m “Second commit”
$ printf “\n\nThird commit” >> file.txt
$ git add file.txt
$ git commit -m “Third commit”
$ git status
$ git log –oneline
$ git reset --soft head~1

Now observe the changes made by the soft reset command

$ status
$ git log --oneline

02 Soft Reset

The following are the results of the soft reset:
1. The third commit has been rolled back as seen in the commit log,
2. No changes have been introduced in the INDEX i.e. the staging area. The git status helpfully suggests that file.txt is ready to be committed into the repository. The contents of the file.txt are the same as they existed prior to the rolled back commit,
3. No changes have been introduced in the working directory. All the contents are intact.

Let’s continue our example to mixed reset:

$ git add file.txt
$ git commit “Third commit”
$ git reset --mixed head~1

Now observe the changes made by the mixed reset command

03 Mixed Reset Cropped

The following are the results of the mixed reset:
1.    The third commit has been rolled back as seen in the commit log,
2.    Changes have been introduced in the staging area, as suggested by the status. It helpfully suggests that file.txt needs to be added in order for it to be committed.
3.    No changes have been introduced in the working directory.

Let’s continue our example to hard reset

$ git add file.txt
$ git commit “Third commit”
$ git reset --hard head~1

Now observe the changes made by the hard reset command

03 Hard Reset Cropped

The following are the results of the hard reset:
1. The third commit has been rolled back as seen in the commit log,
2. INDEX i.e. staging area has been put into the state as it existed after the second commit and prior to the third commit. The data corresponding to the third commit has been lost from the file.txt.
3. Working directory has been put into the state as it existed prior to the third commit. Any contents introduced between the second and the third commit would have been lost as part of the hard reset of the third commit.

Conclusion

Now that we have seen the difference between revert and reset and when the two should be used. We also saw the three modes made available by revert - soft, mixed and hard reset and how they affect the state of the working directory, INDEX and the content. Hard reset should be used with caution since the content is not recoverable after the issuance of the command.

References

1. https://git-scm.com/book/en/v2
2. https://www.atlassian.com/git/tutorials/resetting-checking-out-and-reverting

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